GM 3.6L misfire with no check-engine light

Play original video

Video loads only when you choose to play.

Before the tests: stop repeating the parts list

The written case describes previous plugs, injectors, valve cleaning, intake gaskets, coil swaps and a throttle body without resolving the complaint. We started again with the symptom, a code scan and a visual check. No codes were stored, and the visual check did not identify the cause.

Working through a similar complaint? Use the GM 3.6L diagnostic worksheet to record the vehicle, symptoms, test conditions, findings and next check. Keep an inconclusive result marked as inconclusive.

1. Find the pattern in the scan data

The test drive reproduced the slight misfire. The scan tool counted it on cylinders 1, 3 and 5: bank 1. Long-term fuel trim was about −14% on that bank and nearly zero on bank 2. That contrast focused the investigation on one bank; it did not establish a fuel-system failure or a bad chain.

Misfire counters from the Impala case

Enlarge this capture

Bank 1 fuel-trim capture from the Impala case

Enlarge this capture

2. Keep an inconclusive result open

The next check was relative compression, using a current clamp and an ignition trigger. The result was inconclusive. That left the mechanical question open—it did not clear the engine or justify condemning it. We needed a different look at how the cylinders were breathing.

Relative-compression capture — inconclusive in this case

Enlarge this capture

3. Use firing order to interpret the vacuum pattern

2014 Impala 3.6L firing order: 1–2–3–4–5–6. In this case, cylinders 1, 3 and 5 are bank 1. Reading the vacuum pattern in firing order helps connect every-other-cylinder behavior with the bank already identified in the misfire counters. Confirm cylinder locations from the exact vehicle’s service information before connecting a test lead.

A running intake-vacuum capture showed an alternating pattern: every other peak was higher and every other valley was lower. Combined with the misfire and trim pattern, that made a cam/crank comparison a useful next check.

Intake-vacuum capture — the alternating pattern

Enlarge this capture

4. Compare the timing relationship

For help locating signal landmarks, open the annotated cam/crank waveform references and printable cards. These come from the separate Traverse scope demonstration; confirm engine and test-condition compatibility before using them for your comparison.

The bank 1 cam/crank capture did not line up with the known-good reference used for this case. The article describes the intake crossing about one crank tooth later, with the exhaust relationship also shifted. Bank 2 matched the reference.

What that adds: evidence of a bank 1 timing discrepancy. Match the engine, signals and test conditions before reusing the comparison; this case’s tooth difference is not a universal acceptable limit.

Cam/crank comparison from the Impala case

Enlarge this capture

Where this diagnosis ends

GoTech suspected a stretched bank 1 chain from the combined findings. The car then went back to the referring shop. We do not have a completed repair, an inspected-chain result or a post-repair capture for this Impala. The demonstrated value is the path to the timing finding.

Use the finding to choose the next step—not to skip it.

• Start with the timing-code checks
• See the separate Traverse scope demonstration and reference images
• If mechanical repair is established, plan the replacement